In a post on his blog yesterday, the retired politician expressed scepticism of the prime minister’s popularity. “Najib is very popular, but some recalcitrants may not think so. What really would convince people would be a national referendum, or poll, in which every voter in the country can express his support of Najib,” he wrote.

Malaysia has never held a referendum since independence from Britain.

Throwing down the gauntlet in his post, he promised that “if Najib wins in the national referendum, I will stop asking him to resign”.

The 90-year-old former prime minister has kept up a relentless attack on his former protege.

Earlier this month, he put together a bipartisan alliance to initiate the “Citizens’ Declaration” to pressure Mr Najib to resign.

He remains undeterred, even as Datuk Seri Najib fights back.

Tun Dr Mahathir is even prepared to give speeches from the back of lorries if necessary, said former PAS vice-president Husam Musa at a forum on Sunday night, according to The Malaysian Insider.

He said that while they were discussing plans for a national roadshow, Dr Mahathir had complained that he could not even rent hotel rooms for speeches.

“If Dr Mahathir joins us at our ceramah (lecture) in Kedah, he will speak from the back of a lorry,” Datuk Husam said.